1. Field of the Invention
The present invention relates generally to gypsum compositions for making gypsum containing materials and is also directed to improved methods for making gypsum compositions.
2. Background Art
One of the most common materials used in the construction of walls and barriers is gypsum wallboard, sometimes referred to as “drywall,” or “plaster board.” Wallboard is conventionally produced by sandwiching a core containing an aqueous slurry of calcium sulfate hemihydrate between two sheets of board cover paper. Calcium sulfate hemihydrate is also known as stucco and gypsum. Examples of suitable methods and components for the construction of wallboard can be found in U.S. Pat. No. 6,527,850 , which is incorporated herein by reference.
Gypsum is also used to form many other non-wallboard gypsum products. These include formulated plaster-based powders. Certain segments of the construction systems/building products markets make use of dry-bagged gypsum products composed of redispersible powders. In these instances, premixed gypsum plasters already contain the necessary additives to ensure good workability. Only water need be added by the plasterer. These types of applications include: gypsum concrete or gypsum cement for flowable, self-leveling flooring and subflooring; mortars and grouts for installation, patching, and repair materials; joint treatment compounds; and casting molds.
Gypsum is commonly manufactured by drying, grinding, and calcining natural gypsum rock. The drying step of gypsum manufacture includes passing crude gypsum rock through a rotary kiln to remove any free moisture accumulated in the rock from rain or snow, for example. The dried rock is then passed through a roller mill (a type of pulverizer), wherein the rock is ground to a desired fineness. The dried, ground gypsum is also known to as “land plaster.”
The calcination step is performed by heating the ground gypsum rock, and is described by the following chemical equation:CaSO4.2H2O+heat→CaSO4.½H2O+{fraction (3/2)}H2O
This chemical equation shows that calcium sulfate dihydrate plus heat yields gypsum (calcium sulfate hemihydrate) plus water vapor. This process is conducted in a “calciner,” of which there are several types known in the art. Various methods of producing gypsum are known in the art.
Gypsum is chemically reactive with water, and will “set” rather quickly when the two are mixed together. This setting reaction is a reversal of the above-described chemical reaction performed during the calcination step. The reaction proceeds according to the following equation:CaSO4.½H2O+{fraction (3/2)}H2O→CaSO4.2H2O+heat
In this reaction, the gypsum is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction is generally dependent upon the type of calciner employed and the type of gypsum rock that is used and can be controlled within certain limits by the use of additives such as accelerators and retarders.
In the hydration reaction, hemihydrate gypsum is mixed with water until a suspension is formed that is fluid and workable. The hemihydrate gypsum dissolves until it forms a saturated solution. This saturated solution of hemihydrate is supersaturated with respect to dihydrate gypsum, and so the latter crystallizes out of the solution at suitable nucleation sites. Finally, as the dihydrate gypsum precipitates, the solution is no longer saturated with hemihydrate gypsum, so the hemihydrate gypsum continues to dissolve. Thus the process continues to consume the hemihydrate gypsum. The reaction can be followed by measuring the heat evolved. Initially there is very little reaction and no rise in temperature. This time is referred to as the induction period. As the amount of dihydrate gypsum increases, the mass thickness increases and the material hardens (sets).
In order to facilitate the above reaction and/or provide beneficial properties to the final product, various additives may also be included in the core gypsum slurry. For example, starch, set accelerators and/or set retarders, preservatives, and fiberglass may be included.
As described above, the setting reaction for gypsum involves the reaction of calcium sulfate hemihydrate with water to form calcium sulfate dihydrate. The theoretical (stoichiometric) water content of the slurry required for the reaction of calcium sulfate hemihydrate is about 18.7 weight percent. However, a relatively large amount of water is generally required to provide sufficient fluidity of the calcined gypsum slurry in order to obtain proper flow of the gypsum slurry in the manufacturing process. The amount of water required to provide proper fluidity depends upon various factors, such as the type of gypsum, particle size distribution, the various phases of gypsum in the stucco, source, and the levels of above-described additives conventionally used in minor amounts. This level (amount) of water may be expressed quantitatively as the “consistency”. Consistency is defined as the volume of water required to produce a desired fluidity (flow) for 100 g of gypsum.
Alpha-type gypsum generally requires a consistency of about 34 to about 45 ml of water per 100 grams of gypsum in order to form a readily pourable and flowable gypsum slurry. Beta-type gypsum, on the other hand, typically requires a consistency of about 65 to about 75 ml of water per 100 grams of gypsum.
Certain “water reducing,” “fluidity,” or “consistency-decreasing,” additives/agents have been used in order to improve the fluidity of the above-described gypsum slurry while allowing use of reduced levels of water. Reduction in water usage brings reduced costs in the form of reduced water and energy demands, as less water will have to be removed during the drying step(s). Reduction of water usage also provides environmental benefits.
Various commercially-available fluidity-enhancing, consistency-decreasing, and/or water-reducing agents, i.e., dispersion agents, are known in the art for various applications. Some typical dispersion agents used are calcium lignosulfonate, ammonium lignosulfonate, sodium lignosulfonate, and naphthalene sulfonate. The use of condensation products of naphthalene sulfonic acid and formaldehyde is also known. See also U.S. Pat. No. 4,184,887, the disclosure of which is hereby incorporated herein by reference. Calcium lignosulfonate, ammonium lignosulfonate and sodium lignosulfonate are believed to provide the ability to use reduced water levels, but they retard the set times of gypsum in the hydration reaction discussed above. Setting can be discussed in terms of the “initial” and the “final” set times. The initial set time corresponds to the time at which hydration reaction (setting) begins, while the final set time is the time at which the hydration reaction is completed.
U.S. patent application Ser. No. 2003-0019401-A1, assigned to the assignee of this application, discloses a liquid dispersant that has proven to be particularly useful in gypsum compositions. As many of the materials used with gypsum, and with gypsum itself, being solid, there is a need for solid dispersants that can be used with gypsum.
Accordingly, it would be advantageous to provide a gypsum product manufacturing process that employs the use of a solid consistency-decreasing additive without producing the deleterious set retarding effects found in the prior art. Moreover, it would also be further desirable to provide a gypsum product manufacturing process using a solid consistency-decreasing additive that improves the setting of the gypsum composition.